1. Field of the Invention
The present invention relates to a method of supplying a powder to be added to a resin and a resin extrusion apparatus. More particularly, in the present invention, to produce a resin pellet containing a coloring powder and a resin mixed uniformly with each other, the powder is quantitatively supplied to an intermediate portion of the resin extruder.
2. Description of the Related Art
In producing a resin pellet by mixing coloring powder with a thermoplastic resin serving as the material of the resin pellet, two methods are known. In one method, after the resin and the powder are stirred, the mixture is supplied to a resin extruder. In the other method, after the resin is supplied to the resin extruder, the powder is added to the resin and the powder and resin are mixed with each other.
FIG. 8A shows the case where the resin and the powder are stirred. In this case, after a thermoplastic resin X and a predetermined amount of a coloring powder Y are supplied to a stirrer 2 such as a tumbler and stirred, the resin X and the powder Y are supplied to a resin extruder 1 through a hopper 1a. The resin extruder 1 heats and kneads the resin X and the powder Y and extrudes the mixture from a die 3 disposed at the front end of the extruder 1. After the mixture is cooled in a water tank 4, it was cut to produce a resin pellet P.
FIG. 8B shows the case in which the powder Y is added to the resin X after the resin X is supplied to the extruder. In this case, initially, the thermoplastic resin X is supplied to a resin extruder 1xe2x80x2 from a hopper 1axe2x80x2, whereas the powder Y is supplied to an intermediate portion of the resin extruder 1xe2x80x2 through a screw 6a of a powder feeder 6. Thereafter, the resin extruder 1xe2x80x2 produces a resin pellet P in a manner similar to the above-described manner.
To produce a uniform resin pellet P having non-uniformity in color or the like, it is necessary to quantitatively supply the powder Y to a predetermined amount of the resin X and mix them with each other. In the case where the stirrer 2 is used, the powder Y having a larger weight than the resin X tends to concentrate in the lower part of a stirring chamber 2a when the stirred resin X and the powder Y are supplied to the resin extruder 1. The tendency is conspicuous when a large amount of the powder Y is added to the resin X. Consequently, the amount of the powder Y to the resin extruder 1 is large in an early time period from the start of production and small in the neighborhood of the termination thereof. Thus, there is a variation in the content of the powder Y in the resin pellet P.
In the case where the powder feeder 6 is used, it is impossible to make compensation when there is a difference between a set supply amount of the powder Y and an actual supply amount. This is because the screw 6a of the powder feeder 6 is rotated at a constant speed. Consequently, there is a variation in the specific gravity of the produced resin pellet P and a defective dispersion of the powder Y. Such a disadvantage occurs frequently in the case where barium nitrate and tungsten having a high specific gravity, respectively are used as the powder Y.
As shown in FIG. 9, in Japanese Patent Application Laid-Open No. 60-110411, there is disclosed a powder feeder 6xe2x80x2 which supplies powder to be added to resin quantitatively by using a measuring instrument. The powder feeder 6xe2x80x2 has a storing hopper 7xe2x80x2 provided with a supply mechanism 7axe2x80x2 of cut-gate type, a supply container 6bxe2x80x2 installed on a measuring mechanism 8xe2x80x2, a spring feeder 6cxe2x80x2 whose one end is located inside the supply container 6bxe2x80x2, and a power 6dxe2x80x2 for the spring feeder 6cxe2x80x2. In the powder feeder 6xe2x80x2, the opening and closing of the supply mechanism 7axe2x80x2 of cut-gate type and the rotation of the power 6dxe2x80x2 are controlled according to a numerical value measured by the measuring mechanism 8xe2x80x2.
To supply the powder Y to the resin extruder 1xe2x80x3 by means of the powder feeder 6xe2x80x2, the powder Y is supplied to the storing hopper 7xe2x80x2, the supply mechanism 7axe2x80x2 of cut-gate type is opened and closed appropriately, a required amount of the powder Y is dropped to the supply container 6bxe2x80x2, and a predetermined amount of the powder Y is dropped to a pneumatic feeder 9 by the rotation of the spring feeder 6cxe2x80x2. The pneumatic feeder 9 supplies the resin extruder 1xe2x80x3 with the predetermined amount of the powder Y. The resin X is supplied to the resin extruder 1xe2x80x3 from a hopper 1axe2x80x3. 
The measuring mechanism 8xe2x80x2 measures the weight of the powder Y. Thus, the powder feeder 6xe2x80x2 can supply the powder Y quantitatively to the resin extruder 1xe2x80x3. However, depending on the control system in which the supply mechanism 7axe2x80x2 of cut-gate type and the spring feeder 6cxe2x80x2 are used, there is a possibility that the control system is incapable of following an actual powder supply operation. Thus,there may be a difference between a set supply amount and the actual supply amount. Another disadvantage of the powder feeder 6xe2x80x2 is that the powder Y is supplied quantitatively to the resin extruder 1xe2x80x3, together with the resin X. Thus, the powder feeder 6xe2x80x2 has a problem similar to that of the resin extrusion apparatus using the stirrer 2.
The present invention has been made in view of the above-described problems. Thus, it is an object of the present invention to produce a resin pellet containing resin and powder mixed uniformly with each other by adopting a system of not mixing the resin and the powder with each other beforehand but quantitatively supplying the powder to a resin extruder at an intermediate portion of the resin extruder.
To achieve the object, according to the present invention, there is provided a resin extrusion apparatus including a resin extruder for extruding a thermoplastic resin supplied thereto from one end thereof to the outside from the other end thereof; a screw, driven by a motor, for supplying powder from a powder container to the resin extruder at an intermediate portion thereof, an inverter for setting the number of rotations of the motor for driving the screw; a controller controlling a frequency of the inverter; a weighing device, connected to the controller, for measuring an actual supply amount of the powder in a predetermined time period; and a detector for transmitting a measured value to the controller. Supposing that a set supply amount (g) of the powder in the predetermined time period is A; a tolerance (g) of the set supply amount is B; the actual supply amount (g) of the powder, in the predetermined time period, measured by the weighing device is C; a frequency (Hz) of the inverter is D; a feedback coefficient is E; and a frequency (Hz) of the inverter after a feedback control is executed is F,
when |Axe2x88x92C|xe2x89xa6B, the feedback coefficient E is set in the range of 0.1xe2x89xa6Exe2x89xa60.7 and
when |Axe2x88x92C| greater than B, the feedback coefficient E is set in the range of 0.8xe2x89xa6Exe2x89xa61.2.
Thereby, the powder is quantitatively supplied to an intermediate portion of the resin extruder by means of a rotation of the screw, based on the feedback-controlled inverter frequency:
F=D+Exc3x97{(Axe2x88x92C)xc3x97D/C}.
It is possible to recognize the difference between the set supply amount of the powder and the actual supply amount thereof by detecting the actual supply amount thereof. It is possible to rotate the screw according to a powder supply state and thus supply the powder quantitatively to the resin extruder by making the feedback control such that the difference is compensated. More specifically, in the case where the actual supply amount of the powder becomes smaller than the set supply amount thereof owing to various situations, compensation is made to increase the supply amount of the powder. On the other hands, in the case where the actual supply amount of the powder becomes larger than the set supply amount thereof, compensation is made to decrease the supply amount thereof. In this manner, the supply amount thereof can be so controlled as to approach it to the set value. Measurement is executed at regular intervals in the range of 10 seconds to 180 seconds. The feedback control is executed in correspondence to the measurement. The shorter the measuring time period is, the more accurately the feedback control can be accomplished. The supply position of the powder to the resin extruder is different from that of the resin. That is, the supply position of the powder is located at an intermediate portion of the resin extruder. Thus, it is possible to secure the set supply without the powder and the resin interacting with each other.
Describing the feedback control in detail, the numerical value of the feedback coefficient is set depending on whether the detected measured supply amount C lies within the tolerance of the set supply amount A, and feedback coefficient E and the other set values are substituted into the control equation. In this manner, the powder can be supplied to the resin extruder reliably. The feedback coefficient E and the other set values are set in consideration of the material and amount of the resin and the powder and reflect experimental numerical values. Accordingly, the feedback control can be accomplished in conformity to an actual situation.
The supply amount of the powder is detected by the weighing device. A signal corresponding to the detected measured supply amount of the powder is transmitted to the controller connected to the inverter to execute the feedback control. The use of the weighing device allows the actual supply amount to be measured reliably and the inverter frequency to be set appropriately according to conditions owing to the connection between the controller and the inverter. For example, the weighing device measured the actual supply amount of the powder two to five times successively at regular intervals. The feedback control is executed by using a computed average value of the measured values as the measured supply amount C in each of the measuring time periods. It is possible to execute the feedback control precisely because the feedback control is executed not for each measurement but by computing the average value after measuring the actual supply amount of the powder two to five times successively. To compute the average value, for example, the average value is computed after measurement is executed five times at regular intervals of 15 seconds and then the feedback control is executed to determine the inverter frequency. Thereafter, this operation is repeated and the feedback control is executed at the intervals of 75 seconds .